Tip of the day: to avoid receiving a copy of your own messages sent to a #googlegroup from #fastmail, add a discard rule which tests for existence of "list-id" header and a "from address" with your own address, and then uncheck "do not discard messages sent by my contacts".

Last part is important since you are considered a "known sender" by fastmail filters.

While #protonmail still doesn't officially support Linux with its IMAP/SMTP bridge application (there is a beta, though) and it requires a paid account, there's an interesting open-source third-party bridge called Hydroxide.